Electron discharge apparatus incorporating high frequency resonators



Jan. 21, 1947. H FQE N 2,414,517

ELECTRON DISCHARGE APPARATUS INCORPORATING HIGH FREQUENCY RESONATORS filed June 2, 1942 2 Sheets-Sheet '1 Fig.1.

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mam/role 4, Mu 145w nrme/v'y Patented Jan. 21, 1947 ELECTRON DISGHARGE APPARATUS INCOR- PORATING HIGH FREQUENCY RESONA- TORS John Heaver Fremlin, London W. C. 2, England, assignor to Standard Telephones and Cables Limited," London, England, a British company Application June 2, 1942, Serial No. 445,504 In Great Britain August 2, 1940 8 Claims. (Cl. 250-275) This invention relates to electron discharge apparatus incorporating high frequency resonators of cavity type, the field of which interacts directly with an electron beam.

Various authorities have discussed the forms of electromagnetic oscillation which may take place within totally enclosed spaces with conducting walls. Exact analysis of the field distribution for enclosures of several mathematically simple forms has been made.

Recently these forms of resonator have become of considerable importance as low loss resonating circuits in oscillators and other devices for use at very high frequencies. In many of these applications it is essential that there should be an eificient interaction between an electron beam and the electromagnetic oscillation; the electron beam being transmitted through the resonator by means of relatively small shielded apertures in the wall of the resonator. This requires that maximum electric fields must be developed across distances short compared to the wavelength concerned since even at fairly high voltages electrons are moving at speeds small compared to the velocity of light. Consequently, many of the resonators which are in practice used have re-entrant forms which can only very approximately be calculated.

Th present invention resides in electron discharge apparatus wherein a substantially-closed resonant cavity is formed between two parallel annular conductive surfaces interconnected by two co-axial cylindrical conductive surfaces and wherein means is provided for projecting an electron beam of approximately annular crosssection axially through the cavity in the neighbourhood of the field anti-node in the cavity.

The form of resonator now to be employed is such that its dimensions can be calculated rigorously for any desired wavelength while the whole apparatus is such that the advantages of such a-resonator may be effectively employed.

It is possible to calculate precisely the frequencies for which stationary waves of the transverse electric type can occur between two similar parallel and coaxial annular discs spaced 2. short distance apart. This can be done if radiation at the boundaries of the system is neglected, both for the case when the edges of the discs are free and where a conducting boundary occurs at one or both of the bounding radii.

In Figs. 1 and 2 of th accompanying drawings is shown the shape of enclosure which is formed'if a conducting boundary exists at 110th.

inner and outer radii. Lines of electric force corresponding to the fundamental transverse electric wave are shown. An infinite series of frequencies are possible if the distance between the discs is small, characterised by the number of different radii at which anti-nodes of electric field are obtained, The form of oscillation is not dependent upon the distance between the discs while this distanceis smallrelative to the'wave length of the oscillation concerned.

This form of resonator has considerably less radiation loss than parallel disc resonators having inner or outer edges free. Moreover the position of the field anti-node at an intermediate radius instead of at an extreme radius permits the use of an electron beam of considerable intensity with a high coupling factor upon the resonator field resistance. The electric field lines of the fundamental vibration alone are shown.

The construction of certain forms of oscillators and other devices embodying the invention will now be described with reference to Figs. 3 to 6 of the accompanying drawings.

Firstly it is possible to design a high. power oscillator using a single resonator of the form shown on Figs. 1 and 2. In Fig. 3, a filamentary cathode 3 bent into a circular shape is placed opposite to a slot 4 .in the resonator wall I, arranged at a position of maximum field strength according to the property of the resonator disclosed above. In the far wall 2 of the resonator is cut a circular slot to receive the electrons crossing the resonator'without the emission of secondaries. In the construction shown this far wall is a solid block; it could, however, easily be pressed from sheet metal. The vacuum space is closed by the glass dish 6, through which at a convenient point are taken the leads to the cathode and the shields. Disc 8 between the cathode and the resonator has a circular slot aligned with the cathode and serves as a focussingelectrode. The cathode 3 and disc 8 also appear in Fig. 4 which is a section On line 4-4 of F 3,

Now the transit time of the electrons between th walls I and 2 of the resonator can be adjusted to any desired value by suitable mechanical construction and by applying a suitable voltage between cathode and resonator. It is known that if this transit time is cycles of an oscillationin the resonator the beam will have effectively a negative resistance at the frequency concerned. Being at a field anti-node,

resonator thinner.

power will then be abstracted from the beam and transformed into high frequency power in the resonator. This power may be extracted by any of the known devices used for evacuated cavity resonator oscillators in sealed-oil tubes.

The efiiciency of the device can be improved at the cost of increased complexity of structure without affecting the total output power, by allowing the slot 5 to penetrate completely the wall 2 and by collecting electrons on a final electrode at a suitable potential which can easily be calculated from the known minimum final velocities of electrons in the stream under optimum conditions of oscillation amplitude. In either case it will usually be advantageous to supply some suittable cooling for the electrode which acts as final collector for the electron stream.

Owing to the short distance of travel of the electrons it is not necessary to use a magnetic field to obtain oscillation. It will often be found, however, that the use of a magnetic field Will increase efficiency and when weight is no object it will sometimes be helpful to employ a magnet of the form used in cathode ray tubes, made in two sections to allow assembly. This magnet is shown at l in Fig. 3.

It is not essential that the form of the filament should be exactly circular or that it should follow exactly the circle of maximum field strength. It might for example be made polygonal, in common with the slots, the corners of the polygon being supported and the dimensions so arranged that the maximum current coincided with the maximum field. Alternatively, a series of separate cathodes, indirectly or directly heated With separate holes for the passage of electrons could be used. In such case the separate holes lie in a circular or polygonal line and may be regarded as a discontinuous circular or polygonal slot.

Oscillators may also be made using two resonators having the same advantage of calculabil-- ity and of handling large currents from a ode of considerable extension. In Figs. 5 and 6 is shown a possible construction for an oscillator of this type using two res onators coupled very closely by large holes H in the dividing wall to avoid the need for tuning of one to match the other. Here electrons from the cathode 3 pass first across the gap between the fins l2 in the resonator I 3 where they may be velocity modulated. They then pass along an equipotential space 9 between the resonator l3 and the resonator I!) which space 9 acts as a drift space within which the beam is bunched. Then work is done by the bunches as they cross the gaps in I 0 if the voltage is correctly adjusted.

In this design allowance must be made for the effect of the cylindrical fins in adding to the capacity of the system. The outer pair of fins in each resonator could be eliminated by making the This may be advantageous when the consequently increased capacity/inductance ratio is immaterial.

A system which can be used either as power amplifier or as oscillator can be made by using two resonators loosely coupled together with a thick partition instead of with the spider carrying the fins I 2.

Various modifications and alternatives within the scope of the appended claims will be readily appreciated.

What is claimed is:

1.' An electron discharge device comprising an annularmetallic cavity resonator having parallel annular walls and coaxial cylindrical Walls,

cath- 4 said annular walls being spaced apart a distance less than the operating wave length, one of said annular walls having a concentric aperture therethrough located at a distance from the center at which an electric field anti-node exists,

and an annular cathode disposed "adjacent said' aperture outside of said cavity resonator at a distance therefrom less than the distance between said annular walls.

2. An electron discharge device comprising an annular cavity resonator having parallel annular walls inter-connected by concentric cylindrical walls, said cavity resonator having'an axial dimension between said annular walls substantially less than the operating wave length, one annular wall having a concentric aperture therethrough-at a distance from the center at which an electric field anti-node exists, an annular metal focussing member outside of said cavity resonator disposed adjacent to said apertured annular wall, said focussing member having an aperture adjacent to the aperture in said annular wall, and an annular cathode adjacent said apertured focussing member, said focussing member being separated from said apertured annular wall and cathode by a distance less than the axial dimension of said cavity, said focussing member being adapted to pass a thin annular electron beam for entry through said cavity aperture.

3. The structure of claim 2 wherein said other annular Wall has a concentric channel formed therein opposed to the concentric aperture in said one annular wall.

4. The structure of claim 2 wherein a toroidal magnet is provided for generating a substantially constant magnetic field axially within said cavity resonator along the annular anti-nodal region, said magnet providing complete magnetic shielding for said device.

' 5. An electron discharge device comprising a circular dished glass member having a central press formed therein, leads sealed in said press, a cavity resonator for-med of two parallel annular walls spaced apart a distance less than the operating wave length and inter-connected by two coaxial cylindrical walls, the outer cylindrical wall being extended outside of said cavity and sealed to said dished glass member forming a sealed structure, said resonator having one annular all entirely within said sealed structure and having a narrow concentric aperture therethrough at a distance from the center at which an electric field anti-node exists, and an annular cathode disposed outside of said cavity resonator but within said sealed structure and adjacent said aperture, said cathode being separated from said aperture by a distance less than the distance between annular walls and being connected to said sealed leads.

6. The structure of claim 5 wherein a flat metal focussing plate is provided between said cathode and one annular wall, said focussing plate having an aperture aligned with said aperture in said annular wall, saidfocussing plate being separated from said cathode and one annular wall by a distance substantially smaller than the distance between opposed annular walls.

7. The structure of claim 5 wherein a metallic focussing plate is disposed between said cathode and one annular wall with the focussing plate being separated from said cathode and one annular wall at a distance small in comparison to the distance between opposed annular walls, said focussing plate having an aperture registering with the aperture in said one annular wall, said other annular wall having a concentric channel therein opposed to the concentric aperture in said one annular wall and wherein a toroidal magnet is provided for generating a magnetic field having an annular shape with the lines of force extending parallel to the axis or" said cavity resonator at the anti-nodal region thereof.

8. An electron discharge device comprising two coupled cavity resonators each formed of two parallel annular conducting surfaces spaced apart a distance less than the operating wave 6 length, and inter-connected by two coaxial cylindrical walls, said annular surfaces each having a concentric circular aperture at a distance from the center at which an electric field anti-node exists, said annular conducting surfaces having fins projecting inwardly to the corresponding cavity at the concentric edges of the apertures, and an annular cathode outside of said resonators and disposed adjacent to a concentric 10 aperture.

JOHN HEAVER FREMLIN. 

